FOR IPHONES (FROM YOUR COMPUTER)

Import the ringtone you downloaded into iTunes by either double-clicking on the song in the Finder, or by dragging and dropping it to the iTunes icon in the dock.

Your ringtone should now appear in the Tones section of iTunes. You can preview the ringtone by double-clicking on it just as you would to play any other song in iTunes.

If the ringtone is not importing into the Tones section of iTunes, move the ringtone file in the Finder to the Desktop and remove the 30-second preview song from iTunes. Try the import again. The Tones sidebar item should automatically appear after the tone has been imported; if it doesn't, navigate to iTunes > Preferences > General, and check the "Tones" option.

Sync your phone with iTunes.

After your device is synced, you can select your ringtone by navigating to Settings > Sounds.

(Apple does not provide a way to directly add ringtones to your phone; iPhone ringtones must be downloaded to and installed from iTunes on your computer. You will want to back up any music, movies, TV shows, books and tones on your iPhone before syncing.)